This files are an extraction of the UCAC3 star catalog with stars down to magnitude 16.
They contain only a part of the original catalog data. For each star it include: identifier, RA, Dec, aperture magnitude, proper motion, approximate color index.
You can get only the stars visible for you. i.e. a northern hemisphere observer can get only the North and Equator files.
Only the stars not in Tycho2 and fainter than magnitude 10 are include. So you must always activate the Tycho2 along with this catalog.